JP6652970B2 - Transmission line anti-reflection filter - Google Patents

Description

  The present invention is based on National Science Foundation and Associated Universities, Inc. And with Government support under Cooperative Aggression AST-0223851 between the United States and the United States Government, having certain rights in the invention.

  This application claims priority to US Provisional Application No. 62 / 075,499, filed November 5, 2014, entitled "Transmission Line Reflectionless Filters," and specifically and entirely incorporated herein by reference. It is to assert.

  The present invention relates to electronic filters and methods of their use. In particular, the method relates to non-reflective electronic filters and methods of their use.

  Virtually all electronic systems use some sort of filtering to remove unwanted frequency components. In most conventional filters, the rejected signal is bounced back to the signal source and eventually vanishes in its generator or interconnecting wiring / transmission line or radiates into the instrument box. This manner of rejecting unwanted signals is sometimes due to spurious mixing in nonlinear devices, unintentional rebiasing of sensitive active elements, or crosstalk between various signal paths, sometimes resulting in other elements in the system. Can lead to deleterious interactions with The solution sought a filter that absorbed these unwanted signals before they could degrade performance. This leads to a novel absorbing filter topology patented in 2013 (US Pat. No. 8,392,495), as well as pending US patent application Ser. No. 14 / 724,976, both of which are hereby incorporated by reference in their entirety. Incorporated in the specification. These absorptive filters provide the mixer's sensitivity to poor out-of-band termination, detrimental and unpredictable nonlinear effects from reactive harmonic loading, and capture between the filter and other poorly matched elements. It solves many of the problems encountered with conventional filters, such as leakage or crosstalk due to applied energy, and a number of other problems associated with out-of-band impedance matching. These absorptive filters also provided superior performance and manufacturability compared to other approaches to absorptive filters, such as directional filter structures employing terminated diplexers and orthogonal hybrids.

  However, none of these prior embodiments properly taught how to implement such a design using transmission lines rather than lumped elements. The inability to easily convert absorptive filters to transmission line configurations has limited the frequencies at which they can be effectively implemented in the centimeter wave region and below. Recent efforts to address this problem have provided a practical transmission line solution that easily extends the frequency range to sub-millimeter waves while maintaining the benefits of the original anti-reflection filter topology.

US Patent No. 8,392,495

  The present invention addresses some of the problems and shortcomings associated with conventional filters and the prior art of anti-reflection filters, thereby providing new resources for band selection and definition in electronic systems.

  Embodiments of the present invention are directed to a non-reflective electronic filter. The filter comprises a symmetric two-port circuit, wherein the symmetry defines an even mode equivalent circuit and an odd mode equivalent circuit when the ports are driven in phase and 180 ° apart, respectively, and at least one transmission line; The normalized input impedance of the even mode equivalent circuit is substantially equal to the normalized input admittance of the odd mode equivalent circuit, and the normalized input impedance of the odd mode equivalent circuit is At least one lossy element or a matching internal sub-network disposed within the symmetric two-port circuit to be substantially equal to the normalized input admittance. In this way, the even or odd mode equivalent circuits are said to be duals of each other.

  To design an anti-reflection filter using a transmission line, one can start with a lumped element prototype. Next, the derived circuit is converted into a transmission line form called a network by applying a Richard transformation. Finally, the transmission line identity is used to modify the transmission line network so that it is easier to manufacture in the desired medium (microchip, coplanar waveguide, stripline, coaxial cable, waveguide, etc.). While filter topologies are being developed, the application of these identities is often facilitated by the introduction of matched transmission line sections that are cascaded with intermediate filter structures. This allows certain identity deformations to be used that are not possible with the eigenfilter structure.

  In some embodiments, the resulting network includes a resonator formed by transmission line stubs separated by cascaded transmission lines. In other embodiments, the resonator may be formed by alternating high impedance transmission lines and low impedance transmission lines instead of or in combination with transmission line stubs.

  Note that throughout this specification, the terms "parallel" and "shunt" are used interchangeably with respect to the connection of circuit elements or transmission line stubs.

  The present invention will be described in more detail, by way of example only, and with reference to the accompanying drawings, in which:

FIG. 9 is a diagram showing an even mode equivalent circuit of a prototype antireflection filter using a lumped element together with a matched cascade transmission line of an arbitrary length on the input side, and the order of the filter at this stage is arbitrary. It is a figure showing an even mode equivalent circuit (left) and an odd mode equivalent circuit (right) of a non-reflection filter in an early stage of derivation. It is a figure which shows the even mode equivalent circuit and the odd mode equivalent circuit which incorporated the series short circuit stub and the shunt open stub after the application of Richard conversion. FIG. 4 is a diagram showing an even mode equivalent circuit and an odd mode equivalent circuit after application of Kuroda's identity. It is a figure showing an even mode equivalent circuit and an odd mode equivalent circuit after exchange of a position of a series stub and a termination resistor in an even mode equivalent circuit. It is a figure which shows the even mode equivalent circuit and the odd mode equivalent circuit after the recovery of the symmetry condition near a termination resistor. FIG. 4 shows a complete two-port bandpass anti-reflection filter using a transmission line. FIG. 4 shows a transmission line identity useful for replacing a series connected open stub in a transmission line anti-reflection filter with a coupled transmission line. FIG. 9 illustrates the anti-reflection filter of FIG. 7 after two applications of the identity of FIG. 8; FIG. 10 is a diagram showing a conventional directional filter for comparison with the antireflection filter of FIG. 9. FIG. 3 illustrates one embodiment of a transmission line anti-reflection filter using only a single pair of open stubs. 12 is a plot of the transfer characteristics of the anti-reflection filter of FIG. FIG. 12 illustrates an alternative form of the anti-reflection filter of FIG. 11, wherein the transfer characteristics of the two forms are identical (shown in FIG. 12), but with different coupled line impedance parameters that may be easier to assemble in some cases. Have. FIG. 9 is a diagram showing another embodiment of a transmission line anti-reflection filter using a cascade type line resonator instead of a stub. 15 is a plot of the transfer characteristics of the anti-reflection filter of FIG.

  As embodied and broadly described herein, the disclosure herein provides detailed embodiments of the invention. However, the disclosed embodiments are merely exemplary of the invention, which may be embodied in various and alternative forms. Accordingly, the specific structural and functional details are not intended to be limiting, but rather, to provide principles for the claims and to use the invention in various ways. It is intended to be provided as a representative principle for teaching a trader.

  A problem in the art that can be solved by embodiments of the present invention is circuit topology and design techniques for electronic filters that are well matched at all frequencies. Surprisingly, it has been discovered that such filters have some unexpected advantages, including minimal reflections on their input and output ports in either their passband or stopband, or transition band. Was done. The return loss of these filters is virtually infinite at all frequencies. On the other hand, in conventional filters, stopband rejection is achieved by reflecting unwanted portions of the spectrum toward the signal source rather than absorbing them. Instantaneous filters consist of transmission lines or transmission line equivalents, in addition to lumped element resistors, inductors, and capacitors, and can be implemented in any form suitable for the application (eg, waveguide, coaxial, lead-type). , Surface mount, monolithic integrated type).

  First, start with any symmetric two-port network. Although symmetry is not required for the anti-reflection filter, the preferred embodiment is. In such a network, if both ports are excited simultaneously with equal signal amplitudes and coincident phases, there is no current traversing from one of the symmetry planes to the other. This is called even mode. Similarly, if the two ports are excited with equal amplitudes but 180 degrees apart, all nodes in the plane of symmetry will have zero potential with respect to ground. This is called odd mode.

式中、ｓ_ｉｊはポートｊからポートｉまでの散乱係数であり、Γ_ｅｖｅｎおよびΓ_ｏｄｄは、それぞれ偶モード等価回路および奇モード等価回路の反射係数である。故に、完全な入力整合のための条件、ｓ_１１＝０は、以下のように（１）から得られる：
Γ_ｅｖｅｎ＝−Γ_ｏｄｄ（３） Thus, it is possible to obtain two single-port networks, each containing half of the elements of the original two-port network, where nodes in the plane of symmetry are either open or short to ground. . These may be referred to as an even mode equivalent circuit and an odd mode equivalent circuit, respectively. An equivalent circuit is a circuit that retains all of the electrical properties of the original (often more complex) circuit. The scattering parameters of the original two-port network are then given as a superposition of the reflection coefficients of the even and odd mode equivalent circuits as follows:

_Where s _ij is the scattering coefficient from port j to port i, and Γ _even and ｏ _odd are the reflection coefficients of the even mode equivalent circuit and the odd mode equivalent circuit, respectively. Thus, the condition for perfect input matching, s ₁₁ = 0, is obtained from (1) as follows:
Γ _even = −Γ _odd (3)

This is equivalent to saying that the normalized even mode input impedance is equal to the normalized odd mode input admittance (or vice versa):
z _even = y _odd (4)
_Where z _even is the normalized even mode impedance and y _odd is the normalized odd mode admittance, which is satisfied if the even mode equivalent circuit and the odd mode circuit are duals of each other (eg, , The inductor is replaced by a capacitor, and the shunt connection is replaced by a series connection). Furthermore, by combining (2) and (3), the transfer function of the original two-port network is directly given by the even mode reflection coefficient:
s ₂₁ = Γ _even (5)

  Therefore, it is often useful to configure the even mode equivalent circuit as a dual of the odd mode equivalent circuit, and vice versa. When the filter includes a transmission line, the dual replaces the cascaded transmission line with another having a normalized inverse characteristic impedance, and replaces open stubs with short-circuit stubs and parallel connections with series connections. May be configured. In some embodiments, transmission line identities may need to be applied to restore symmetry after configuring the even and odd mode equivalent circuits, or to make the topology more easily manufacturable. . In a preferred embodiment, the Kuroda identity is particularly useful for transforming a series connected stub into a parallel connected stub or vice versa. To enable this particular identity deformation, it is often the case that one or more matched cascaded transmission lines are inserted at the input and / or at the loss termination of the even or odd mode equivalent circuit. Note that it is useful.

  In some preferred embodiments, it is useful to apply a transmission line identity that replaces a cascaded transmission line with a transmission line stub at one end with a coupled transmission line. In other embodiments, the series connected stub may be swapped with a loss termination connected in series with it, resulting in a series loss element followed by a parallel connected stub.

  Note that the anti-reflection filter, including the transmission line, can be enhanced with matched internal sub-networks. These sub-networks themselves include transmission lines, lumped elements, or both.

  In a preferred embodiment, the non-reflective bandpass electronic filter including the transmission line is preferably designed as follows: First, an even mode equivalent circuit is terminated high-pass including a ladder network of series inductors and shunt capacitors. It is drawn as a pass filter. It has previously been shown that the transfer characteristics of a symmetric two-port network are equal to the reflection characteristics of an even mode equivalent circuit. Furthermore, when replacing the lumped element in the even mode equivalent circuit with a transmission line, the high pass response is converted to a band rejection response as a result of the periodicity of the transmission line scattering parameters. To facilitate the application after identity deformation, insert a matched transmission line in cascade at the beginning of the even mode equivalent circuit at this stage (this will affect the reflection phase of the circuit but affect the amplitude response). Not useful). Thus, the resulting even mode equivalent circuit is in FIG. Although a third-order filter is shown in this example, the order of the filter is arbitrary at this stage. The odd mode equivalent circuit is preferably configured as a dual of the even mode equivalent circuit, ie, by replacing the series element with a shunt element, replacing the shunt element with a series element, replacing the inductor with a capacitor, and replacing the capacitor with an inductor. The resistive termination and the matched input transmission line area remain unchanged. The resulting even mode and odd mode equivalent circuits are shown in FIG. Next, a well-known Richard transformation is preferably applied to transform the reactive element into a transmission line stub. The result is shown in FIG. Modifications are preferably made to both even and odd mode equivalent circuits to restore the assumed symmetry of the filter without changing the port behavior, where for the first time these modifications are Not just for networks with transmission lines.

  In a preferred embodiment, Kuroda's identity is used to transform the series shorted stubs on both sides into open shunt stubs at quarter wavelength intervals, as shown in FIG. One series short stub remains unchanged at the end of the even mode equivalent circuit.

  Next, in the preferred embodiment, the positions of the remaining short-circuit stubs and termination resistors in the even-mode equivalent circuit that are connected in series are exchanged. This leaves the short stub now in the shunt position, as shown in FIG. A connection line is preferably drawn from the node between this shunt short-circuit stub and the terminating resistor in the even mode equivalent circuit to the plane of symmetry. In an odd mode equivalent circuit, the ground node of the termination resistor is preferably replaced by a virtual short on the plane of symmetry, and the shunt short stub is preferably attached to this virtual ground node. This completes the restoration of symmetry near the termination resistor, as shown in FIG.

  To restore symmetry near the port node in a preferred embodiment, an open shunt stub on the input side of the odd mode equivalent circuit is connected in series between the input node and the virtual ground of the plane of symmetry. Similarly, a series open stub is preferably added between the input node of the even mode equivalent circuit and the plane of symmetry. At this point, a complete two-port anti-reflection filter that satisfies all symmetry and duality conditions is obtained, as shown in FIG. However, series open stubs are not feasible with some transmission line media. In a preferred embodiment, the transmission line identity shown in FIG. 8 can be applied to eliminate these series open stubs. The resulting network uses a coupled transmission line and is shown in FIG.

  At this stage, as shown in FIG. 10, it is beneficial to compare this topology with a more conventional and well-known type of absorbing filter. An input and output quadrature hybrid, called a directional filter, directs reflections from the two sub-filters to a terminating resistor at either end. Quadrature hybrids are often approximated using coupled transmission lines. However, this is not an anti-reflection filter because the amplitude and phase balance between the through port and the coupling port only provide good impedance matching over a limited range of frequencies close to 0 dB and 90 degrees, respectively. . On the other hand, the anti-reflection filter embodiment of FIG. 11 uses a similar set of elements, but is well matched at all frequencies, including those with arbitrarily large amplitude and phase imbalance in the coupled line section.

The order of the filter selected in the first even mode equivalent circuit determines the number of open stubs in the final transmission line network. This number is arbitrary. In one embodiment, only a single pair of open stubs is required, as shown in FIG. The characteristic impedance of transmission lines and stubs is constrained by duality and symmetry requirements. They can be parameterized with respect to a free parameter, x> 1, as follows:

  The bandpass transfer characteristics of this circuit are shown in FIG. The reflection response of this circuit is equally zero at all frequencies.

An alternative to the anti-reflection filter may be obtained by first adding another matched transmission line segment at the input port (equal to a shift in the port reference plane) before applying the transmission line identity in FIG. The identity is then applied to these new transmission lines with series stubs, essentially reversing the orientation of the coupled lines in the resulting circuit (shown in FIG. 13). This leaves an additional cascade area labeled "Z _x" in FIG. The characteristic impedance is again constrained by duality and symmetry requirements as follows:

  The resulting filter has exactly the same impedance and transfer characteristics as that of FIG. 11, but has different coupled line parameters that may be easier to assemble in some situations. As before, the order of the filters and the resulting number of transmission line stubs is arbitrary.

In previous embodiments, the filter resonator was formed by a transmission line stub. In other embodiments, one or more of the transmission line stubs may be replaced by additional cascaded transmission lines. In a preferred embodiment, these additional cascaded transmission lines are:
^{_{Z r = Z 0 x -1 (}} 8)
Has a characteristic impedance given by

Examples which may be obtained by replacing the stub resonator Z _oc in FIG. 11 and line resonator Z _r is shown in FIG. 14. Its simulated performance is shown in FIG. In general, cascaded line resonators provide lower sidelobes at the price of a more rounded passband corner.

  Other embodiments and uses of the invention will be apparent to those skilled in the art from consideration of the specification and practice of the invention disclosed herein. All references cited herein, including United States and foreign patents and patent applications, are specifically and entirely incorporated by reference. It is intended that the specification and examples be considered as exemplary only, with a true scope and spirit of the invention being indicated by the following claims. Furthermore, the term “comprising of” includes “consisting of” and “consisting essentially of”.

Claims (15)

A non-reflective electronic filter,
A symmetric two-port circuit, wherein the symmetry defines an even mode equivalent circuit and an odd mode equivalent circuit when the ports are driven in phase and 180 degrees apart, respectively;
At least one transmission line;
The normalized input impedance of the even mode equivalent circuit is substantially equal to the normalized input admittance of the odd mode equivalent circuit, and the normalized input impedance of the odd mode equivalent circuit is At least one loss element or matching internal sub-network disposed within the symmetric two-port circuit to be substantially equal to the normalized input admittance ;
A coupled transmission line section connected to the first port and a coupled transmission line section connected to the second port ;
Even mode equivalent circuit or the odd mode equivalent circuit, a reflecting filter and heat transmission line that is cascaded,
Each loss element or matched internal subnetwork provides a matched termination in the stopband signal path of the anti-reflective electronic filter;
Each through node of the coupled line section is connected to the same chain of cascaded transmission lines and transmission line stubs;
A chain of cascaded transmission lines and transmission line stubs is terminated by two terminating resistors connected together or in a matched internal sub-network having a common node on both sides;
A node joining the two terminating resistors or a common node of the matching internal sub-network further attaches at least one paralleling stub;
The cascade transmission line, the transmission line stub, and the coupled transmission line are all essentially one quarter wavelength long at a center frequency of operation.
Non-reflective electronic filter. The anti-reflection filter according to claim 1, wherein the anti-reflection filter further includes at least one transmission line stub. 3. The anti-reflection filter according to claim 2, wherein the at least one transmission line stub is one of a short-circuit type and an open type. 3. The anti-reflection filter according to claim 2, wherein the at least one transmission line stub is one of a parallel connection and a series connection. The anti-reflection filter according to claim 1 , wherein the anti-reflection filter comprises at least one cascaded transmission line resonator . The anti-reflection filter according to claim 1, wherein the anti-reflection filter further includes a coupled transmission line. The anti-reflection filter according to any one of claims 1 to 6 , wherein the anti-reflection filter is one of low-pass, high-pass, band-pass, band rejection, and multi-band. Subnetworks, the transmission line network, or lumped element network or a combination of a transmission line and lumped elements, non-reflective filter according to any one of claims 1 to 7,. The stubs in the chain of cascaded transmission lines and transmission line stubs are open,
At least one parallel connection stub is attached to the common node of the node or matching internal subnetwork joining two termination resistors are short-circuited, non-reflective filter according to claim 1. The open stub

Where Z _oc is the characteristic impedance of the open stub, Z ₀ is the characteristic impedance of the system, x is the free parameter,
The short-circuit stub
^{_{Z sc = Z 0 (x-}} x -1)
And Z _sc is the characteristic impedance of the short-circuit stub,
A continuous cascaded transmission line with no stub in between,
Z _high = Z ₀ x
Z _low = Z ₀ x ⁻¹
, The high characteristic impedance and the low characteristic impedance are alternately repeated,
Here, Z _high is the characteristic impedance of the high impedance cascade transmission line, Z _low is the characteristic impedance of the low impedance cascade transmission line,
The free parameter is x> 1, and
Resistance R of the resistor is equal to the characteristic impedance of the system, which is R = Z _0, no reflection filter according to claim 9. The total number of cascaded transmission lines and transmission line stubs in each chain is odd;
The first element in each chain connected to the through port in the coupled line area is either a low impedance, Z _low , stub or cascaded line;
The backward wave coupling port of the coupling line is connected,
The forward wave coupling port of the coupling line is opened,
The coupled line impedance parameter is

Where Z _even is the even mode characteristic impedance of the coupled line , Z _odd is the odd mode characteristic impedance of the coupled line, and ρ is the even mode characteristic impedance and the odd mode characteristic of the coupled line. The anti-reflection filter according to claim 10 , which is a ratio of impedance . The total number of cascaded transmission lines and transmission line stubs in each chain is even,
The first element in each chain connected to the through port in the coupled line area is a high impedance, Z _high , cascaded line;
The backward wave coupling port of the coupling line is opened,
The forward wave coupling port of the coupling line is connected,
The coupled line impedance parameter is

Where Z _even is the even mode characteristic impedance of the coupled line , Z _odd is the odd mode characteristic impedance of the coupled line, and ρ is the even mode characteristic impedance and the odd mode characteristic of the coupled line. The anti-reflection filter according to claim 10 , which is a ratio of impedance . The anti-reflection filter of claim 11 , wherein each chain includes only a single open stub. The anti-reflection filter of claim 11 , wherein each chain includes only a single cascaded transmission line. 13. The anti-reflection filter of claim 12 , wherein each chain includes only a single cascaded transmission line and a single open stub.

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